This invention relates to a process for the detoxification of aqueous solutions which contain cyanohydrins, in particular glycolonitrile, and/or nitriles and in addition contain decomposition catalysts for peroxide compounds, in particular manganese compounds, by perhydrolysis of the cyano compounds with peroxide compounds.
Effluents containing cyanides and/or cyanohydrins or nitriles may be detoxified by the process according to U.S. Pat. No. 3,970,554 comprising the addition of peroxide compounds at temperatures of from 10.degree. to 80.degree. C. and pH values of from 6 to 12 in the presence of iodide ions and optionally silver ions. Hydrogen peroxide is mentioned as the preferred peroxide compound. Although perborates, percarbonates and peroxides are also described as effective in the said document, a comparison of Test 10 (Example 2) with Test 25 (Example 7) shows the superiority of hydrogen peroxide over perborate, since the perborate required three times as long to lower the cyanide content to below 0.1 mg/l.
It has been found that effluents containing glycolo nitrile, such as those obtained, for example, when exhaust gases containing hydrogen cyanide are scrubbed in the presence of formaldehyde, cannot be satisfactorily detoxified with hydrogen peroxide if manganese compounds are present at the same time. In the metallurgical industry, the process according to EP-B 0 223 904 is used for the purification of blast furnace gases. In this process, formaldehyde is introduced into the gas scrubbing circuit to form glycolo nitrile and the glycolonitrile formed is subjected to perhydrolysis by means of hydrogen peroxide. In the treatment of the wash waters from the production of ferromanganese, vigorous decomposition of hydrogen peroxide takes place due to the presence of the manganese in the wash water so that complete perhydrolysis cannot be achieved or only by using very large excesses of H.sub.2 O.sub.2. Under these conditions, however, it is questionable whether the process of purification of exhaust gas/effluent is economical and the officially allowed limits of toxic materials may not even be achieved. The previously known detoxication of aqueous solutions containing cyanohydrins and/or nitriles may be impaired in its effectiveness and economical efficiency not only by Mn compounds but also by the presence of other decomposition catalysts for peroxide compounds, in particular copper compounds and active charcoal.
In the process of "perhydrolysis", the cyanohydrins and nitriles are converted into the non-toxic hydroxycarboxylic acids or carboxylic acids from which the cyanohydrins or nitriles are derived, for example glycolo nitrile is converted into a hydroxy-acetamid and glycollic acid. The hydroxycarboxylic acids and carboxylic acids may be further decomposed in known manner by peroxide compounds or by biological degradation. The cyanohydrin content in aqueous solutions may be determined by means of cyanide determination processes in which the cyanohydrin is decomposed into the carbonyl compounds and HCN and the latter is removed from the equilibrium for determination of the cyanide, for example by alkalization to a pH&gt;12 and argentometric titration with potentiometric indication of the end point. The nitriles may be determined by, for example, chromatographic processes.